This regiment was a garrison force based at Liaoyang and Jiamusi in Manchukuo until June 1940, when it was transferred to Hubei in central China.

In August 1940, Iwasa was promoted to major general and assigned to the staff of the IJA 9th Division, back in Machukuo.

In April 1942, Iwasa was appointed commander of the IJA 3rd Independent Garrison Unit, but from December 1942, he was reassigned to the staff of the Japanese Eighth Area Army in the Japanese-occupied Solomon Islands and New Guinea.

In June 1943, Iwasa was appointed commander of the IJA 65th Infantry Brigade, responsible for the defenses of Rabaul.

During the Battle of Bougainville, Iwasa successfully opposed the landings of the United States Marine Corps during the Koiari Raid.